The structural landscape of ferrocenyl polychalcogenides
Introduction
Over the past two decades, we have seen an ever-growing interest to the halogen bonds (HaB) [1], chalcogen (ChB) [2], [3], [4], [5], [6] and other σ-hole interactions [7]. All these emerging non-classic interactions, although different in their specificity and directionality, can be considered as the extension of classic covalent and hypervalent interactions or a part of continuum from covalent to non-covalent bonding [8], and therefore link the molecular and supramolecular design. In this interdisciplinary field, the organometallic chemistry meets crystal engineering, providing not only versatile building blocks for the hybrid cocrystals, but also the models for the understanding of the mechanisms of supramolecular self-organization and crystal genesis. The classic organometallics ferrocene derivatives are among them. Varying the nature of chalcogens in 1,1’-ferrocenyl tri-chalcogenides, Karjalainen et al. studied their structural landscape using single-component systems and demonstrated how the degree of specific chalcogen-chalcogen intermolecular association increases with the increasing content of heavy chalcogens [9]. In our recent work, a series of ferrocene cocrystals with triptycene were used to markup the structural landscape of the latter [10]. In general, the co-crystallization strategy used in the latter work was quite efficient for the active study of the structural landscapes [11,12]. Therefore in this project, we used the halogen-bond (HaB)-donor coformer (1,4-diiodotetrafluorobenzene, p-DITFB) as the co-crystallization partners for ferrocenyl polychalcogenides (HaB acceptors respectively) varying from a rather compact ferrocenophane triselenide 1,1’-FcSe3, to bulky diferrocenyl ditelluride Fc2Te2 and its mixed valent derivative Fc2Te2I2, bearing the polar TeI2 group (see Fig. 1). In addition to the HaB-accepting functionality of chalcogen(II) atoms in these species, we can expect additional stabilization of the HaB-assisted cocrystals by the π-π interactions between the π–hole of DITFB with the electron-rich Cp-ring of ferrocenyl fragment.
Comparison of the molecular packing in the parent crystals with the supramolecular organization in the cocrystal can reveal certain consistent packing patterns, which are typical to a given type of molecules and can be transferred intact from the native crystal to cocrystal [13,14]. The existence of such robust molecular aggregates can be confirmed using the intermolecular energy calculations (TONTO/Crystal Explorer) and energy framework analysis [15]. The other natural outcome of such analysis is finding the weak links between these robust aggregates in the native crystal. From the practical point of cocrystal structure prediction, these week links indicate the sites in the molecular packing, which are more prone to the intervention of respective coformer molecules and substitute these weak links by more specific and strong intermolecular interactions. Therefore energy framework analysis of the starting crystal can suggest some constructive ideas on the packing pattern of the cocrystal it may form with a certain co-crystallization partner. In general, this offers a unique way to visualize and predict the pattern of supramolecular interactions [16]. With this in mind, in addition to the co-crystallization of p-DITFB with the range of ferrocenyl-polychalcogenides (see Fig. 1), we have also analyzed the energy framework of their native crystals in order to attain some ideas about the possible packing pattern of their cocrystals with HaB donors.
Section snippets
FeSe3 •p-DITFB cocrystal
Relatively compact 1,2,3-triselena[3]ferrocenophane (later indicated as FcSe3) is known in two polymorphic forms, namely Pca21 [17] and P21/c [18]. Although both of them feature a few intermolecular Se···Se contacts, these interactions are not structure-determining as compared to its telluride-containing congeners [9] (see Figs. 2 and S2).
Therefore, in the absence of pronounced aggregates in the native crystals of FcSe3, one can hardly expect the fragments of their packing patterns to reappear
Conclusion
Halogen bond assisted co-crystallization strategy appeared suitable and efficient for the exploration of the structural landscape of ferrocenic polychalcogenides. Three different patterns: 1) conservation/elongation of the chain structures for FcTeTeI2Fc, 2) transformation of single Se···Se intermolecular bonding to double for FcSe3, and 3) for Fc2Te2, in the absence of specific and directed intermolecular interactions, the cocrystal packing of Fc2Te2 is governed only by p-DITFB HaBs
Experimental part
All the synthetic manipulations were carried out in absolute solvents. Commercial 1,4-DITFB was used without further purification. Fc2Te2I2 was prepared in situ, FcSe3 [26,29] and Fc2Te2 [26,29] were prepared by published methods.
SC-XRD
Suitable X-ray quality crystals of 1-3 were obtained directly during the co-crystallization experiments. A Bruker APEX II CCD area detector diffractometer equipped with a low-temperature attachment was used for the cell determination and intensity data collection. Structures were solved by direct methods and refined by means of least squares method for F2 in anisotropic (isotropic for H atoms) approximation in SHELXTL and Olex2 software [30,31]. Positions of Н atoms were calculated
Declaration of Competing Interest
There are no competing interests to declare.
Acknowledgment
This work is supported by Russian Scientific Foundation (grant № 19-13-00338). XRD experiments were performed using the equipment of shared experimental facilities supported by the Kurnakov Institute of General and Inorganic Chemistry RAS.
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